An important type of modern dental restorative consists of an organic polymer containing carboxylic acid groups combined with an inorganic filler material which is normally a calcium fluoroaluminosilicate glass powder. Cations within the glass react with the ionized carboxylic acid groups of the polymer to form salt-bridges. Such salt bridge formation results in the polymer/glass mixture becoming a hardened, insoluble matrix. Important properties of such dental restoratives are high levels of strength, toughness and wear resistance, and also minimal shrinkage.
The polymer component of the restorative is perhaps most critical for imparting high quality mechanical and minimal shrinkage properties to the dental restorative. One method of improving both the mechanical and shrinkage properties of the polymer has been to graft a methacrylate onto the polymer backbone. Such methacrylates are attached to the polymer backbone via the carboxylate moieties. Examples of graftable methacrylates which have been used for this purpose include 2-isocyanatoethyl methacrylate (IEM) and glycidyl methacrylate (GM).
Acidic polymers which have methacrylates grafted onto their backbone are visible light-curable (VLC) polymers. Typically, the acidic polymer is a copolymer of acrylic acid with itaconic acid. However, copolymers of acrylic acid and maleic acid are also known. The polyacid is made visible light-curable by the reaction of isocyanoethyl methacrylate (IEM).
The reaction of an acid-containing polymer (structure I below shows a polymer of acrylic acid and itaconic acid) with IEM (structure II below) to give the VLC polymer (structure III below) is performed in the organic solvent tetrahydrofuran (THF). 
In addition to the VLC polymer, the other major components of the dental restorative, i.e., the resin modified glass ionomer (RMGI), are a basic aluminofluorosilicate glass, 2-hydroxyethyl methacrylate (HEMA), water and a visible light polymerization initiator. The structure of HEMA is shown below (structure IV). 
The RMGI is a dual cure system which involves (1) an acid-base reaction between the ionized carboxylate groups of the polymer and the basic glass, and (2) a copolymerization of the methacrylate groups present on the major components, i.e., the methacrylate groups which are pendant on the VLC polymer, the HEMA methacrylate groups, and in some cases, methacrylate groups which are attached to the glass. A strong visible-light source is used to catalyze formation of the covalent bonds. Hardening of such dental restoratives occurs due to the two types of bonding. Some products also contain a redox initiator system which will further cure the methacrylate groups after the light source is removed. These are referred to as tri-cure systems.
Unfortunately, IEM, one of the methacrylates which is commonly used to prepare the VLC polymer, is a highly toxic liquid with significant regulatory barriers to its manufacture and distribution on a large scale. IEM also presents health and environmental concerns. In addition, IEM is very expensive and, thus, adds significantly to the cost of preparing VLC polymers. Reactions which employ GM as the methacrylate are high temperature reactions that are conducted at temperatures in excess of 90xc2x0 C., and can lead to premature polymerization of methacrylate groups.
Accordingly, it is desirable to have new methods which do not utilize IEM or GM to prepare the VLC polymers which are used in resin modified glass ionomers.
The present invention provides new methods for preparing free radical or visible light curable (VLC) acid-containing polymers in an aqueous solution. Such polymers are prepared from a polymer backbone containing reactive carboxylic acid groups. In one embodiment of the invention, the method of making the free-radical or visible light curable polymer comprises reacting polymers having carboxylic acid groups with a methacrylated oxazoline or oxazine, collectively referred to hereafter as xe2x80x9cmethacrylated unsaturated cyclic imino ethers,xe2x80x9d in an aqueous solution at a temperature of from about 50xc2x0 C. to about 75xc2x0 C. In another embodiment, the method comprises an additional step of preparing the carboxylic acid-containing polymer in an aqueous solution and then reacting the carboxylic acid-containing polymer with the methacrylated cyclic imino ether in the aqueous solution (i.e., one-pot, two-step reaction). A general scheme for reaction of a copolymer of acrylic acid and itaconic acid monomeric units (structure V) with an N-oxazoline (structure VI) to produce a VLC polymer (structure VII) is shown below. 
n=2 or 3
X=O, NH or NR (R=alkyl residue)
Rxe2x80x2=aliphatic and/or aromatic residues, possibly containing heteroatoms such as N, O or S
The present invention also relates to VLC acid-containing polymers that are made in accordance with the present methods, and dental restoratives, or RMGI, that comprise such polymers.
The present invention also relates to a method of attaching an oxazoline or oxazine to a carboxylate group on a polymer in an aqueous solution.